The present invention relates to high-strength alumina porcelain compositions for electric insulators, and more particularly to such compositions of the kind comprising a non-plastic portion of calcined alumina, a plastic portion of clays and kaolins, and a flux portion of feldspars and micaceous minerals, for firing at temperatures between 1,250.degree. and 1,400.degree. C. These compositions are used especially in or as large and complicated electric insulators.
Alumina porcelains have gained in importance in recent years on account of their considerably superior strength, in comparison with quartz porcelain, especially in the field of high-voltage insulation. Nevertheless, still relatively littie is known as to the phase constitution of alumina porcelains and the formation and solution processes of their various components. This appears from a report by R. Stabenow and H. W. Hennicke "Inquiries Into Phase Constitution, Structure and Mechanical Properties of Alumina Procelain", Keramische Zeitschrift (1976), pages 227 to 229. There it is mentioned that the proportion of glass has hardly any influence upon the strength, but that a rising proportion of corundum appreciably increases the strength. Also included is a discussion of the significance of the pore structure for the strength of the alumina porcelain. One investigated composition contained 23% by weight of corundum in the form of calcined alumina, with about 8% by weight coming from the clay portion and 20% by weight from the flux portion in the form of feldspar and nepheline syenite, so that the total Al.sub.2 O.sub.3 content in the body was about 51% by weight. Furthermore, it is known from Ceramic Bulletin, Vol. 40 (1961), pages 44 to 77, to replace feldspar, which is introduced from orthoclase or albite into alumina compositions, entirely or partially by alkali-containing fluxes such as nepheline syenite, and to use additions of manganese dioxide and wollastonite.
By reason of its high Na.sub.2 O content, nepheline syenite leads to an aggressive flux action and shortens the sintering interval. Correspondingly, the durability and distortion resistance during firing are reduced. Fired materials with this constituent have a higher proportion of glass phase and more pores, thus resulting in poorer mechanical and electric strength.
The use of feldspars, clays and calcined alumina for the production of electric insulators is likewise known from German Pat. No. 1,571,372, and U.S. Pat. No. 3,686,007 in which an alumina porcelain is described having a composition of 15 to 45% by weight of calcined alumina, 30 to 60% by weight of clays and kaolins, and a flux portion of &lt;20% by weight. The flux portion consists of feldspar and nepheline syenite and 0.5 to 4% by weight of TiO.sub.2 -MnO.sub.2. Fluxes such as manganese dioxide and especially titanium dioxide, when present in aluminum-containing compositions, even with small additions of about 0.2% by weight, as also occur as impurities in unusable clays and kaolins, lead to a drastic restriction of the sintering interval with the above-stated consequences. Furthermore, both oxides react sensitively in a reducing kiln atmosphere, as is usual in ceramic firing. This forms the oxygen-poorer, semi-conductive oxides Ti.sub.2 O.sub.3 and TiO which additionally uncontrollably reinforce the flux effect and reduce the electric insulation capacity.
Porcelain compositions, especially with a content of above 40% by weight of calcined alumina, have not in practice satisfied all technological and electro-mechanical requirements. Since the calcined alumina represents a non-plastic constituent, difficulties arise as regards the deformability of the composition, especially in the case of large and complicated insulators. On the other hand, a high proportion of flux leads to a narrow sintering range. Thus, the danger exists of the composition becoming too soft, and deforming under the action of its own weight, in the firing of the products. If, on the other hand, the proportion of flux is too low, the sintering process does not progress sufficiently, so that a product with the desired structure cannot be achieved. In extreme cases, one obtains partially porous articles which do not conform with the required electrical and mechanical properties. As appears from the prior art, hitherto it has been sought to solve this problem in various ways, without however recognizing the full significance of the proportion of flux.